1. Field of the Invention
This invention pertains to the art of making foamed low density polyethylene plastic of relatively large thickness and relatively large lateral cross-section and having a density of about 0.6 to about 1.5 pounds per cubic foot (pcf). More specifically, this invention pertains to the art of making ultra low density foamed plastic boards, planks, beams and the like of substantial dimensions measuring several inches thick and wide and a few to several feet long used primarily for cushioning, packaging, building insulation, flotation, sound deadening and other utilities.
2. Description of the Prior Art
Heretofore, polyolefin foamed plastics in the form of planks, beams, etc. were produced using a variety of blowing agents. For example, it has been common practice to use chlorofluorocarbons (CFC's) and, more recently, hydrochlorofluorocarbons (HCFC's) as blowing agent. Such blowing agents have been found to deplete the planet Earth's ozone layer which serves as a shield to protect the planet from cancer-promoting ultraviolet radiation. Governmental pressure is mounting to severely restrict the release of CFC's and HCFC's to the atmosphere in order to slow down or stop the depletion of the ozone layer.
There is great environmental and commercial interest in the elimination of all ozone-depleting chemicals used as blowing agents. While HCFC's have a lower ozone depletion potential (ODP) than CFC's, the level is still unacceptably high and therefore the use of HCFC's is also coming under greater regulation. It is possible to make polyolefin foam using light hydrocarbons such as isobutane which, however, leads to a worsening of physical properties, e.g., a decrease in thermal stability.
U.S. Pat. No. 3,067,147 (Rubens et al) discloses the production of polyethylene foam having a density as low as 1.7 pcf using 1,2-dichloro-tetrafluoroethane. There is no disclosure of isobutane as a blowing agent or of any stability control agent. This patent discloses that other hydrocarbon blowing agents produce foams having non-uniform large cells.
U.S. Pat. No. 3,379,802 (Raley et al) and U.S. Pat. No. 3,766,099 (Kawai et al) disclose polyethylene foams containing synthetic elastomers but fail to disclose polyolefin foams having densities as low as about 0.06 pcf. The lowest density foam disclosed by Raley et al is 1.7 pcf and the lowest density foam disclosed by Kawai et al is 1.68 pcf. Kawai et al fails to utilize polystyrene or a stability control agent or an isobutane blowing agent and Raley et al fails to disclose the utilization of polystyrene and a stability control agent.
U.S. Pat. Nos. 4,370,378 and 4,387,169 (Zabrocki et al) disclose polyethylene foams alleged to have densities of 1.1 to 1.88 pcf. The Zabrocki et al patents assert that the low densities are achievable by special mixing procedures. There is no disclosure in these patents of using an elastomer, polystyrene, a stability control agent or isobutane. To the contrary, the patents urge the use of ozone-depleting blowing agents such as the CFC's.
U.S. Pat. Nos. 4,640,933, 4,663,361 and 4,694,027 (Park) disclose polyolefin foams having densities between 2.19 pcf and 2.66 pcf but fail to disclose polyolefin foams of lower densities or the use of a synthetic elastomer or the achievement of densities as low as about 0.6 pcf.
U.S. Pat. No. 4,652,590 (Hoki et al) discloses the manufacture of low density polyethylene foams using CFC blowing agents. The patent fails to disclose the use of a synthetic elastomer or a stability control agent or isobutane as blowing agent.
U.S. Pat. Nos. 3,893,957, 4,721,591, 4,738,810 and 4,824,720 each disclose the manufacture of polyolefin foams but fail to disclose the use of isobutane or other hydrocarbon blowing agent or the use of stability control agents.
U.S. Pat. No. 4,379,859 (Hirosawa et al) discloses prefoamed particles of polypropylene produced by dispersing the resin particles in water followed by release into the atmosphere and drying.